Many surgical procedures, particularly those used in orthopaedic surgery, require holes to be drilled in a bone of a patient. Surgical drills have long been used for this purpose, and various mechanical guidance instruments exist to enable a surgeon to accurately drill a hole to a correct depth and without danger of damaging the surrounding tissue. Such drill guides often provide a visual depth gauge which requires the surgeon to read the depth of the drilled hole off a graduated scale-on the instrument.
U.S. Pat. No. 5,895,389 issued Apr. 20, 1999 to Schenk et al. discloses such a drilling guide and measuring instrument. The guide generally comprises a sleeve and a plunger that telescopes within the sleeve, both having axial bores adapted to receive and guide a portion of a drilling tool that protrudes beyond a drill chuck. A plurality of fingers on the forward end of the plunger are biased inwardly by an inside wall of the sleeve, providing a frictional, sliding fit. Therefore the plunger and sleeve retain their relative telescopic position after they have been partially collapsed. Gradations on the side of the plunger indicate the relative movement of the plunger with respect to the sleeve, and therefore the penetration depth of the drill. The sleeve preferably has a small handle for controlling the drilling guide. An alignment bore extends through the handle in parallel with the central axis of the sleeve and plunger bores. By sliding the handle with the alignment bore over a guide wire pre-inserted into the workpiece, the drilling guide may be aligned with the parallel wire at a predetermined distance therefrom. 
With the advent and growing use of computer aided surgery (CAS), much greater accuracy is possible for many surgical procedures. Surgeons can now plot on a computer generated 3D model of the patient, before the actual procedure, the ideal location, orientation and depth of a drill hole, for example. During the surgery, position of the instruments with respect to scanned images of the body parts can be displayed on monitors to guide the surgeon during the procedure.
One CAS system currently employed comprises the use of at least two cameras, located at different stationary reference points, which simultaneously record the location of a moving three point axis. Knowing the positions of the reference points, the unique position in space of the three point axis, and therefore any object to which the axis is fixed, is uniquely defined and can therefore be precisely tracked.
While such instrumentation tracking systems work well for some applications, problems nevertheless exist with certain current surgical uses of visually tracked systems. In order for the cameras to record accurate simultaneous images of the three point tracker axis, and for the location of the points of the axis to be correspondingly precisely computed, the visual images of the axis must remain relatively distortion free. As such, any displacement of the tracker axis with respect to the surgical tool to which it is fastened, results in inaccurate calculation of the exact three dimensional (3D) spatial position, and therefore inaccurate representation on the computer monitors of the operative instrument with respect to the patient. Therefore, the attachment brackets and fixation adapters for securely locating the tracker axis to the tool are often complex.
CAS systems have been employed in conjunction with a surgical drill to attempt to monitor the location and depth of holes drilled into the bone of a patient for such surgical procedures as pin implantation and prosthesis fixation. For such applications, CAS three point tracker axis have been fastened directly to the drill.
A major problem associated with current attachment methods for fixing a CAS tracker axis to a drilling tool, is that many hospitals use significantly different drill  systems. Therefore fastening a CAS tracker axis to each type of drill requires many parts and a completely different set of complex fixation adapters in every case. This necessitates a custom installation for fixing a CAS tracker axis to each and every type of surgical drill, thereby adding considerable expense to CAS systems which already represent a significant expenditure for hospitals. Additionally, the added bracketry required to sufficiently fix the CAS tracker axis to the drill, causes an unnecessary reduction in the freedom of movement that the surgeon has to manipulate the drill.
Therefore, accurate real time visual or electromagnetic tracking of instruments used to drill holes during surgeries has been so far been impractical and expensive for widespread use with all types of surgical drilling systems.